PROJECT SUMMARY Gap junctions are essential for maintaining rapid, synchronous, electrical coupling of cardiac muscle cells. This electrical coupling facilitates the healthy heart beat. Connexin43 (Cx43, gene name GJA1) is the predominant ventricular gap junction protein, and loss of Cx43 gap junction coupling leads to the arrhythmias of sudden cardiac death. The GJA1 mRNA undergoes internal translation to yield a truncated Cx43 isoform capable of modulating gap junctional coupling. The role of this truncated Cx43 isoform, GJA1-20k, in gap junction maintenance and the mechanism by which it regulates gap junctional coupling is unknown. The proposed research aims to provide insight into the role of internal translation and GJA1-20k in Cx43 gap junction formation, and how this process is regulated in the cell by addressing the following Aims; Aim 1: Test if TGF-? induced alteration in translation of GJA1 is sufficient to limit gap junction formation. Though promotion of internal translation and thereby GJA1-20k increase gap junctional coupling, the mechanism of this regulation is unknown. A cellular model of gap junction disassembly will be defined and applied to further study the role of GJA1-20k in maintenance of Cx43 gap junctions at the cell membrane. Aim 2: Determine if ribosomal protein s25 is necessary for internal translation of the GJA1 RNA and GJA1-20k expression.Levels of GJA1-20k are subject to dynamic regulation by the cell, indicating a mechanism by which the cell can alter the translational program. Ribosome specialization is a potential mechanism by which internal translation may be governed, and the ribosomal protein rps25 is a candidate driver of internal translation. The ability of rps25 containing ribosomes to promote internal translation of GJA1- 20k will be studied to define the role of rps25 in regulating internal translation. The proposed research will define the role of GJA1-20k in modulating gap junctional coupling and describe a mechanism of its regulation, identifying potential therapeutic targets for restoration of gap junctions in the failing heart.